Review of hex Boron Nitride coating

Following discussions of hex Boron Nitride coatings, and especially the question of particle size to use, three bullets were examined under an electron microscope to see how the coating looked after coating.  These included an uncoated bullet, a Tubb bullet with hBN coating, and a bullet coated in a home tumbling process.

The coating process involves taking hBN powder, steel balls, and bullets, and tumbling them together.  As the steel balls impact the copper, it impacts the hBN onto the surface of the copper, making it stick.

The conclusion of the examination is that the mechanism for the hBN sticking to the copper is that the hBN particles are mashed into a paste which mechanically locks into any surface imperfections in the copper.  The particles in the paste probably stay attached to each other due to molecular Van der Walls forces, in the same way that tablets, like asprin, can be formed by compressing powder under high pressure.  The individual particles of hBN 'paste' eventually overlap, forming a continuous hBN layer, meaning the starting particle size for the hBN is probably not important.  Differences seen between the two coated bullets suggests that Tubb is achieving a more continuous coating, as islands of uncoated copper are visible on the home coated bullet.  This might be remedied by tumbling longer for the home process.

What you are seeing:

Each bullet was imaged at 250x, as well as 2,500x.

The images are titles culo and cuhi for copper, low and high magnification, tubblo and tubbhi for the Tubb low and high magnification, and homelo and homehi for the home coated low and high magnifications.  Images in this page a jpg format for faster loading.  If you click on the image, you will download the uncompressed high quality image in TIF format.  These are large files - about 5 meg each, so this might take a while if you are on dial-up.

Due to contrast adjustments, the copper surface appears light colored in the uncoated bullet, while appearing dark in the home coated bullet.  Thus, the dark patches on the home coated images represent uncoated copper, while the light areas are the hBN coating.

Non impacted particles of hBN can be seen on the surface of both bullets, it appears these particles are around 2um in size.